Method for determining the position of cement slurry in a well bore



Sept. 5, 1939- A. L. ARMENTROUT E-r AL METHOD FOR DETERMINING THEPOSITION OF CEMENT SLURRY IN A WELL BORE Filed oct'.v 25, 1957 ssheets-sheet 1 .e v u" w Sept 5 1939- A. ARMENTROUT Er A1. 2,171,840

s sheets-sheet 2 MmN.WMM,

Filed Oct. 25, 1957 METHOD FOR DETERMINING THE POSITION OF CEMENT SLURRYIN A WELL BORE l V 'A' l 47 ANV- 4Q5" ui 43 60 6/ Sept 5, 1939. A.ARMENTROUT Er Al. 2,171,840

METHOD FOR DETERMINING THE POSITION OF CEMENT SLURRY IN A WELL BOREFiled Oct. 25, 1937 3 Sheets-Sheet 3 @//Q www Patented Sept. 5, 1939UNITED STATES 2,171,840 METHOD FOR DETERMININ G THE POSITION OF CERIENTSLURRY IN A WELL BORE Arthur L. Armentrout and Ray Gudie, Los Angoles,Calif., assignors to Baggah Corporation, Los Angeles, Calif., acorporation of California Application October 25, 1937, Serial No.170,862

11 Claims.

This invention relates to `well drilling and well drilling equipment,and relates more particularly to a method for determining the locationof a body of cement slurry in a well bore at the time of installationand before the cement slurry hardens or attains its initial set. Ageneral object of this invention is to provide a rapid commerciallypractical method for accurately determining the location of a body ofcement slurry in a well.

In the course of drilling and bringing in deep wells such as oil wellsand gas wells it is necessary to seal oif any unwanted id or substancesuch as water from the interior of the well casing so the oil or gas canbe produced free and uncontaminated from any foreign substance. 'I'hisis done by cementing the casing in the well to provide a cement seal orshut oil' around the lower portion of the casing which excludes thewater, etc. The cement slurry generally used in cementing operations inwell bore usually consists of a homogeneous mixture of approximately 50%to '70% high grade Portland cement and from 30% to 50% water. A waterbearing stratum at the lower end of a well is sometimes sealed oil bywhat is known as a bottom plug which is a body of cement introduced inthe lower portion of the well to prevent the entrance of the water intothe well. Cement is-also employed to seal oil leaks in the well casingand for various other purposes. In carrying out such cementingoperations it has been the usual practice to estimate the quantity ofcement required by cal- -culating the theoretical size or volume of thespace to be occupied by the cement and then introducing this volume ofcement into the Well. In many instances the actual size or capacity ofthe space that receives the cement is much greater than estimated withthe result that. the cement job is a failure or is incomplete. Thepresent methods employed for testing a cement job are costly and timeconsuming and cannot be made for several days after the introduction ofthe cement slurry or until it sets or hardens.

- In some instances it is necessary to repeatedly introduce a charge ofcement slurry into the well, allow the charge to harden and then testthe seal, and these repeated operations greatly delay the completion ofthe well and materially add to the cost of the operations.

Another object of this invention is to provide a method for accuratelydetermining the position of the upper surface or the lower surface, orboth, of a body of cement slurry in a well at the time of theintroduction of the cement slurry in the well or immediately thereafter.The method of the present invention provides for an accuratedetermination of the position of a body of cement slurry in the Well,or, if desired, the vertical dimension of the body of cement -be rapidlycarried out without slurry at the time of its introduction orimmediately thereafter, whereby the operator may be immediately madeaware of the necessity for additional cement slurry to complete thecementing operations and have a knowledge of the location of the cementslurry and, thereafter, a knowledge of the eiiiciency of the seal formedby the cement slurry without resorting to further tests.

Another object of this invention is to provide a method of the charactermentioned that may employing expensive or special cementing equipment.

A further object of this invention is to provide a method of thecharacter mentioned that may be employed to accurately locate the cementy slurry in the well when the cement slurry is introduced by any of theusual methods and when employed to provide seals for special purposes.

'I'he various objects and features of our invention will be fullyunderstood from the following detailed description of typical preferredforms of apparatus and manners of carrying out the method of theinvention, throughout which 'description reference is made to theaccompanying drawings, in which:

Fig. 1 is a longitudinal sectional view of a typical well, illustratinga body of cement slurry passing downwardly in the well casing andshowing one form of apparatus employed by the invention arranged to makea test. Fig. 2 is a View similar to Fig. 1 illustrating the body ofcement slurry passing the electrode unit. Fig. 3 is a view similar toFig. 1 showing the upper surface of the body of the cement slurry at oradjacent the electrode unit. Fig. 4 is a diagrammatic view of theapparatus illustrated in Figs. 1, 2 and 3 showing the electrodesubmerged in the upper surface portion of the body of cement slurry.Fig. 5 is a view similar to Fig. 4 illustrating another form ofapparatus. Fig. 6 is a view similar to Fig. 5 showing still another formof apparatus. Fig. '7 is a wiring diagram of the apparatus illustratedin Fig. 5. Fig. 8 is a wiring diagram of the transmitter of theapparatus illustrated in Fig.6 and Figs. 9, 10 and 11 are longitudinalsectional views of a well illustrating the manner of employing themethod and apparatus of the invention to test a bottom plug.

'I'he method of the present invention may be employed to locate theposition of a body of cement slurry in a well when carrying onpractically any type of well cementing operation, and the method issuited for use in conjunction with various types of well cementingequipment. In the following detailed description we will describeseveral forms of apparatus employed in connection with typical wellequipment and typical well cementing apparatus to carry out the methodof the invention for the purpose of determining the the casing III.

location of cement slurry in the well. It is to be understood that theinvention is not to be construed as limited or restricted to thespecific forms of the apparatus or the particular applications of themethod about to be described, but that it is to be taken as includingany features or modifications that may fall within the scope of theclaims.

Figs. 1 to 4, inclusive, of the drawings illustrate one form of theapparatus employed to determine the location of a body of cement slurryC in a well during the course of cementing the lower portion of a casingI0 in the well to ei'- fect a shut-off or to provide a seal about thecasing. It is believed that the method employed in this particularsituation will be best understood following a disclosure o'f the wellequipment, the cementing equipment and the said apparatus and we willproceed with a detailed description of these elements.

The well equipment and the cementing equipment illustrated in Figs. 1 to4, inclusive, of the drawings comprises the casing III lining the wellW. The casing III projects downwardly into an open uncased lower portionof the well. A cement shoe or float shoe S is provided on the lower endof the casing III and has a spring held valve II operable to close oilthe lower end of the casing against the entrance of the well fluid sothat the casing may be floated into the well. The shoe S and the valveII are frangible so that they may be drilled up following the cementingoperation. A cement head H is provided on the upper end of The head H isprovided with pipes I2 for the admission and discharge of fluid and tofacilitate the introduction of the cement into the well. In accordance:with the usual practice the casing I0 is suspended from its upper end tohave its lower end a short distance above the 'bottom of the well W. Itwill be assumed 'that the well W contains water or the rotary mud Musually employed in connection with the rotary method of well drilling.

The form of apparatus illustrated in Figs. 1 to 4, inclusive, of thedrawings includes, generally, a conductor or line I4 to be run into thewell W, an electrode unit I5 on the lower end of the line I4, means I6for measuring the line I4 to indicate the position of the unit I5 in thewell, and an electrical circuit I1 providing a resistance gap at theelectrode unit I5.

'I'he line I4 is provided to carry or suspend the electrode unit I5 'andto carry the electrical current to the unit. The line I4 is preferably awell insulated electrical cable or conductor adapted to be exposed totheiluids in the well W without shorting. 'Ihe line I4 is ilexible to bereadily handled or reeled up and, if desired, may be marked withcalibrations of length to facilitate its measurement. In use the line I4is passed through a suitable gland I8 on the upper end of the head H toextend downwardly through the casing I II.

'I'he electrode unit I5 is intended to be runfinto the well onthe lineI4 to locate the cement slurry C or to determine the depth or thevertical dimension of the body of cement slurry in the well. The unit I5preferably includes a weight or a body I9 secured to the lower end ofthe line I4 and one or two electrodes carried by the weight I9 andexposed to the fluid in the well. 'I'he weight I9 may be a simple,cylindrical member formed of lead or other material o1' relatively highspecific gravity and of suillcient weight to carry the line I 4 into thewell and to maintain the trode 2II is electrically connected with theelectri- 5 cal conducting line I4 and projects from the lower end of theweight I9 to be exposed to the fluid in the well or in the casing Il.When the unit I5 is provided with a single electrode 20 the current iscarried by the line I4 and the casing I0 and therefore there is a.resistance gap between the electrode 20 and the casing Il. It isdesirable to maintain the electrode 20 substantially centralized in thecasing III so that the resistance gap between the electrode 2l and thecasing is substantially uniform. Insulated guide ns 2| are provided onthe weight I9 and are engageable with the casing III to maintain theunit I5" and its electrode 20 centralized in the casing.

Various methods and means may be employed to measure the line I4 for thepurpose of determining the position of the electrode unit I5. It ispreferred, however, to employ the means I6 for this purpose. The linemeasuring means I6 is in the nature of a line reeling mechanism providedwith a measuring device. As illustrated in the vdrawings the means I6includes a winding drum or reel 22 on which the line I4 is wound. Acounterbalanced pivoted lever 23 is supported on the frame 24 of thereel 22 and carries a 8 and into the casing I0. The pulley 25 is urged uagainst the line I4 by its own weight and the weight of the associatedparts to be rotated by the line. A revolution counter or meter 21 isdriven by the pulley 25. The meter 21 is calibrated to indicate thelength of the line I4 in the well or more properly to indicate theposition of the electrode unit I5 in the well.

'Ihe circuit I1 is provided to produce and maintain a resistance gap atthe electrode unit I5 in and through the fluid in which the unit issubmerged. In the form of the apparatus illustrated in Figs. 1 to 4,inclusive, of the drawings where the unit I5 has only one electrode 20the casing III forms a part of the circuit I'I, that is, one lead 28 ofthe circuit I1 extends from a battery 29 or other suitable source ofelectrical power to the casing I0. A second lead 30 extends from theother terminal of the battery 29 to the reel 22 where it is electricallyconnected with the'fline I4,V through the line I4`t'o theelectrode 20and then through the fluid in the spacepr gapgbetween the electrode andthe surrounding casing I0. The fluid in the well or the casing I0between the electrode 20 and the casing ,thus forms a reoil sistor orresistance in the circuit I1. A suitable galvanometer or resistancemeter 32 is connected in the circuit I'I to indicate the resistance inthe Acircuit. In practice the meter 32 may be interposed in the leadIII. As the resistance in the 6I The circuit I1 vis completed 54 slurryhave widely different electrical resistances. In the event that therotary mud M or water in the well is weighted with borium, ferrousmaterials, or the like, so that its electrical conductivity iscomparable to that of the cement slurry C it is preferable to introducea quantity of fresh water into the casing C ahead of the cement slurryas well as a quantity of fresh Water to immediately follow the body ofcement slurry through the casing, thiswater forming a stratum ofrelatively high resistance immediately above and below the body ofcement slurry.

Accordingly, movement of the electrode 20 from the rotary mud M or freshwater into the cement slurry C, or vice versa, will be accompanied by amarked change in the resistance in the circuit II, which sudden changewill be indicated by the meter 32. much higher conductivity than typicalrotary mud or fresh water and in the event that the electrode unit I5 ismoved, or the cement slurry C is moved past the unit I5, so that the mudM or fresh water in the resistance gap between the electrode 20 and thecasing Cl is replaced by the cement slurry the meter 32 immediatelygives an appreciably lower reading.

We shall assume in the following explanation and description that therotary mud M or water has a higher resistance than the cement slurry.

In employing the method of the invention with the apparatus illustratedin Figs. 1 to 4, inclusive, `of the drawings and for the purpose ofdetermining the position of the body of cement slurry C in the well, themethod includes, generally, the positioning of the electrode unit I5 ata selected point in the well or in the casing I0, the delivery orintroduction of a body of cement slurry C into the casing IIJ to encasethe lower portion of the casing, and the reading of the meter 32 inconjunction with the meter 21 to note changes in the resistance in thecircuit I1 when the cement slurry C moves past the said point.

The electrode unit I5 may be run into the well or the casing I Il on theline Il at or immediately following the application of the cement head Hto the well preparatory to the cementing operation. In one manner ofcarrying out the method of the invention the unit I5 is lowered throughthe casing I0 to the plane to be occupied by the upper surface of thebody of cement slurry C. For example, in Fig. 1 of the drawings it maybe assumed that the electrode unit I5 has been lowered to a plane to beoccupied by the upper surface of the body of cement that is to encasethe lower end part of the casing I0. The exact position of the electrodeunit I5 may be determined by the meter 21 and the reel 22 may beoperated to arrange the unit I5 in the selected position. It may beassumed that the well Wis initially occupied by water or the rotary mudM so that when the unit I5 is lowered to the position illustrated inFig. 1 of the drawings it is submerged in the mud M or water. 'I'heresistance in the circuit I'I may be noted at this time by reading themeter 32.

The cement slurry C may be introduced in the casing I in the usualmanner, for example, it may be pumped through a pipe I2 of the head H tomove down in the casing C as an integral mass. It may be desired ornecessary to force the body of cement slurry C downwardly through thecasing IU and its shoe S under pump pressure. The quantity of cementintroduced in the casing IIl may be determined or calculated in theusual Cement slurry has a passed the electrode unit I manner. 'I'he massor body of cement slurry C pumped downwardly through the casing I Il, asdescribed above, displaces the mud M or Water from below it and movesdownwardly to surround the electrode unit I5. The body of slurry C isfollowed by a column of Water or mud M. When the cement slurry Creplaces the mud M or water in the resistance gap between the electrode20 and the casing I0 the resistance in the circuit I'I drops and thischange in resistance is indicated by the meter 32. Thus the operator ismade aware of the fact that the body of cement slurry C has reached theelectrode unit I5. The mass or body of cement slurry C flows downwardlythrough the shoe S and upwardly around the lower portion of the casingIII.

When the upper surface of the mass or body of cement slurry C reachesand passes the electrode 20' so that the electrode is again contacted bythe mud M or the above mentioned water, the resistance in the circuit Ilsuddenly increases and this change is made apparent by the meter 32.This indicates to the operator that the entire mass or body of thecement slurry C has and is in the correct position. In some instances itis necessary to determine where the upper or lower surface of the bodyof the cement slurry C is in the well or casing. The upper or lowersurface of the cement may be accurately located by merely raising orlowering the unit I5 on the line I4 until the electrode 20 moves out ofthe cement slurry and into the mud M or the water above or below thecement, at which time the resistance in the circuit I'I immediatelyincreases and the meter 32 gives an indication of the change in thecircuit resistance which reading may be taken with the reading of themeter 2l, the meter 2'I indicating the position of the electrode 20 inthe well at the time that it passes from the cement slurry C into themud M or water and therefore indicates the exact position of either theupper or lower surface of the cement slurry. It will be understood thatif the upper surface of the body of cement slurry C is found to be belowthe point desired additional cement slurry may be pumped or passed intothe casing C to raise the upper surface of the body of cement slurry tothe desired plane. To determine the exact quantity of cement slurry C inthe casing I0 or to determine the vertical dimension of the cementslurry in the casing I0 it is only necessary to move the electrode unitI5 on the line I4 vertically through the mass of slurry from its uppersurface to the shoe S. Upon the entrance of the electrode 20 into thecement slurry C the resistance in the circuit I'I drops so that themeters 32 and 21 indicate the depth of the upper surface of the cementand when the unit I5 engages the shoe S the tension on the line I4 isimmediately reduced and the meter 2'I will indicate the bottom of thecolumn of cement slurry C in the casing I0. In most instances theposition of the shoe S is known and it is only necessary to determinethe position of the upper surface of the column of cement slurry Cstanding in the casing to determine the quantity of cement in the casingat the end of the cementing operation.

Figs. 5 and 7 of the drawings illustrate another form of apparatus andanother manner of carrying out the method of the invention in which amarked variation in the inductance of a coil associated with a vacuumtube transmitter unit is indicated by a meteror earphones associatedwith a receiver at the ground surface to locate the upper or the lowersurfaces, or both, of a body of cement slurry in the well.

The apparatus illustrated in Figs. 5 and 'I of the drawings comprises anelectrical conducting line |4*1,a'l metering reel 22a handling the line|4, an oscillating circuit transmitter 35 on the line I4a having aninduction coil 36 exposed t0 thc iluid in the Well or casing |8 and areceiver 31 at the ground surface for indicating a change in thefrequency when the induction of the coil 36 is varied by the characterof the material in its field.

The line |4 and the reel 22'L may be the same as the line |4 and theline 22 described above. The reel 22 is equipped with a metering pulley25* equipped with a meter 21e. The line I4* may be considered as passingfrom the metering pulley 25a through a head at the upper end of thecasing I8 and into the casing.

The transmitter 35 is in the nature of a vacuum tube oscillating circuitprotected against the fluids in the well or casing I8 and having theinduction coil 36 exposed to the fluids so that its field is occupied bythe fluid in the casing I8. The various elements of the circuit ortransmitter 35 with the exception of the coil 36 may be housed in a case38 secured to the lower end of. the line |41. The case 38 fully protectsthese various elements against the fluids in the well. The coil 36 maybe carried on the lowerend of the case 38 to have its winding orwindings exposed to the fluids in the casing I8. It is to be understoodthat the circuit of the transmitter 35 may be varied as desired or foundpractical, and that the particular circuit illustrated diagrammaticallyin the drawings is not to be taken as restricting the invention.

The transmitter circuit shown includes a vacuum tube 39, a high voltagelead 48 for the plate of the tube 39, and a heater circuit for the tube39 comprising a lead 4| anda lead 42. The leads 48, 4| and 42 supplyingcurrent to the tube 39 may be considered as extending through theinsulated line I4* to the surface of the ground where they are connectedwith suitable batteries A and B. The lead 4| is connected with oneterminakof the coil 36. A line 43 extends from the other terminal of thecoil 36 to the grid of the vacuum tube 39. A choke coil 44 is connectedin the plate lead 48 and a by-pass condenser 45 is connected between theleads 48 and 4|. A condenser 46 is provided in the lead 43 and aresistor 41 is connected between the leads 4| and 43 to provide thedesired grid bias voltage. The receiver 31 of the apparatus is intendedto be arranged at the upper end of the well and has means for indicatingor recording the variations in frequency resulting from variations inthe inductance of the coil 36. The circuit of the receiver 31 may bevaried as found necessary and the particular receiver illustrated in thedrawings is not to be taken as restricting the invention. The more orless typical receiver 31 shown in the drawings includes a vacuum tube58, a line 5| carrying current to the plate of the vacuum tube, a line52 carrying current to the cathode of the vacuum tube and a line 53connected with the grid. The lines 5| and 52 are connected with thebatteries A and B. An ammeter or micro-ammeter 54 is interposed in theline 5| to indicate the plate voltage and earphones 55 may also beconnected in the line 5| to facilitate the reception of the signals. Ifdesired an oscilloscope may be connected in the line 5| in place of themeter 54 or in addition to the meter 54 to produce a record of thesignal changes. The grid circuit of the tube 58 is suitably biased by avariable condenser 56 and rev sistors 51 connected between the lines 52and 53 and a condenser 58 in the line 53. The receiver 31 is tuned tobeat against the transmitter 35.

In carrying out the method of the invention with ther apparatusillustrated in- Figs. 5 and 7 of the 'drawings the transmitter 35 ishandled by the line I4* which in turn is handled by the reel 22* in thesame manner as the electrode unit I5 described above. It is believedthat' it will be apparent how the line I4 carrying the case 38 and theinduction coil 3)6 of the transmitter 35 may be manipulated in thewell'to locate the position of a body of cement slurry C in the well.

Assuming that the transmitter 35 is being lowered y on the line |4, asshown in Fig. 5 of the drawings, so that the coil 36. is moving from themud M or water into the cement slurry C, the mud or water in the fieldof thecoil is being replaced by the cement slurry. As the character ofthe material in the field of the coil 36 governs the induction of thecoil and as the inductance of the coil governs the frequency of thesignals transmitted it will be evident that movement of the coil 36 fromthe mud M into the cement slurry C will produce a change in thefrequency which may be noted by the meter 54 or by the earphones 55, orthat may be ,recorded by the oscilloscope when the latter is employed.In the event that the mass or body of the cement slurry C moves past orJbeyond the coil 36 or the coil 36 is drawn out of the cement slurry intothe mud, the cement slurry in the eld of the coil will be replaced bythe mud or waterto change the inductance of the coil and thereby changethe frequency in the transmitter and receiver circuit, which change willbe indicated by the meter 54 and may be heard by the earphones 55. Thereadings of the 'meter 54 and the changes in the signal noted by meansof the earphones are taken together with the readings of the line meter21* to accurately determine the position of the induction coil 36 at thetime of the change in its inductance., 'I'hus the exact position of thebody of cement slurry C may be accurately determined or, if desired, theelevation of the upperor lower surface of the body of cement slurry C,and, therefore, the height of the column of the slurry may be easily andquickly determined with the apparatus.

Figs. 6 and 8 of the drawingsillustrate a third form of apparatusemployed in carrying out the -method of the invention in determining thelocation of a body of cement slurry C in a well. In the apparatusillustrated in Figs. 6 and 8 the capacity or the power factor of thecondenser 68 exposed to the fluid in the well or in the well casing I8,is measured or recorded and is considered in connection with the readingof the meter 21* to determine the exact location of the body of cementslurry.l

In this form of apparatus the above mentioned condenser 68 may comprisetwo spaced plates 6l connected through a low capacity circuit to arecording amplifier which in turn is connected to a direct readingcapacity bridge at the surface of the/ground. In the apparatus justmentioned the "dielectric or material between the plates 6| of thecondenser. 68 determines capacity of the condenser and, therefore,determines the reading at the bridge and when the condenser 68 is movedout of the cement slurry C into the ing or forcing the rotary mud M orwater or is moved from the into the cement slurry the capacity of thecondenser 60 is varied and the bridge indicates or records a change inthe capactaken together with the reading of the line meter 2'!aindicates the position of the body of cement slurry in the well.

In actual practice it may be preferred to directly connect the plates 6iof the condenser 60 with a transmitter circuit or a vacuum tubeamplifier. As illustrated in the drawings the plates I of the condenser6U are connected with the poles of the coil 36 of the transmitter 35described above and illustrated diagrammatically in Fig. 7 of thedrawings. 'I'he plates 6I are arranged to be exposed at the lower end ofthe case 38 of the transmitter 35 so that the iluid or material in thewell or casing I 0 forms the dielectric between the plates.

In employing the apparatus illustrated in Figs.

6 and 8 of the drawings the line I4a carrying the transmitter 35 ishandled by the reel 22a and may be moved or manipulated in the casing I0in any desired manner. As the dielectric constant" of water or mud andthe cement slurry C markedly diier the`capacity 60 is materially changedwhen the line I4al is manipulated to move the condenser 60 from the mudM or water into the cement slurry or from the cement slurry into the mudM or water. This change in the capacity of the condenser 60 varies thesignal of the receiver 31 or changes the reading of the bridge at theground surface. The transmitter 35 provided with the exposed condenser60 may be employed in the same manner as the above described electrodeunit I5 to determine the exact position of the upper surface or thelower surface, or both, of a body of cement slurry C in a well or todetermine the height of a column ol cement slurry in a well or a wellcasing.

Figs. 9, 10 and 11 of the drawings illustrate the manner in which themethod of this invention may be employed to determine the progress in abottom plug cementing job and to test the suiciency and character of theoperation at the time of the introduction of the cement slurry.

Fig. 9 illustrates a portion of a typical well W' whose lower portionenters a water bearing sand or stratum 70. A stratum Il of shale islocated above the water bearing stratum 10 and a producing sand or oilbearing stratum 12 is above the `.shale stratum Il. As is often the casein situations of this character the lower portion of the well in thestratum 'l0 is enlarged in size or diameter and a calculation of thecapacity of this portion of the Well obtained by considering thediameter of the well bore and the vertical dimension of the portion ofthe bore in the stratum 'I0 will be inaccurate. In order to close oifthe water bearing stratum 10 and exclude its water from the well W amass or body of cement slurry C' is introduced into the well to occupythe portion of the well in the stratum 10. This may be readilyaccomplished by passcement slurry downwardly through a pipe P. 'Ihe pipeP may extend from adjacent the bottom of the well W to the surface ofrthe ground and the cement slurry may be passed downwardly through it inany desired manner to discharge into the enlarged lower portion of thewell bore. As the quantity of cement slurry C required can only beestimated by calculating the theoretical capacity of the portion of theWell W' entering the stratum C the upper surface of the body of cementslurry C' introduced into the well may be at a plane such as indicatedat X in Fig. 10 of the drawings when the cement has been delivered tothe well.

An electrode unit I5 may then be lowered through the pipe P on the lineI4 to determine the location of the upper surface of the body of cementslurry C. In Fig. 11 of the drawings we have illustrated an electrodeunit I 5' provided with two spaced electrodes 20 similar to theelectrode 20 described above.v The electrodes 20' are arranged so thatthere is a definite resistance gap between them that is occupied by thefluid in the well. The line M', the means I6 and the electrode unit I5may be identical with the precorresponding elements illustrated in Figs.1 to 4, inclusive, of the drawings. The testing circuit I1 connectedwith the electrodes 2U may be the same as the above described circuit I1except that its two main leads are connected with the two electrodes20'. It is to be understood that the apparatus illustrated in Figs. 1 to4, inclusive, the apparatus illustrated in Figs. 5 and 7, or the Figs. 6and 8.may be employed in connection with the bottom plug cementingoperation illustrated in Figs. 9, 10, and 11, if desired, and that theseapparatuses are manipulated in sub` stantially the same manner to obtainthe desired information.

Assuming that the initial charge of cement slurry C introduced into thewell has its upper surface at the plane X, the line I4 is run into thewell until the electrodes 20 enter the body of cement slurry. Theentrance of the electrodes 20 into the cement slurry C' is accompaniedby a variation in the reading of the meter 32 of the circuit I1 and thischange meter 32' taken together with the reading of the line meter 21indicates the depth or position cement slurry is then passed or pumpeddownwardly through the pipe P and when this operation has been completedthe line I4' is manipulated to bring the electrode unit I5 to a positionwhere its electrodes 20' enter the upper portion of the cement slurry C.The point of entrance of the electrodes 20 into the cement slurry C isnoted by into account the readings of the meters and if it is found thatthe upper surface of the cement slurry C' is above the Water sand or thewater bearing stratum 'l0 the cementing operation is completed. Fig. 11of the drawings discloses the electrode unit I5 in a position where itselectrodes 2U' are entering the upper surface portion of the body ofcement slurry C and shows the cement slurry C' completely occupying theportion of the well in the stratum 10 and occupying a part of the wellbore passing through the stratum 1| so that the water sand It is to beparticularly noted that the method of the present invention provided foran accurate determination of the upper surface of the cement slurry C'at the time of, or immediately following, the introduction of the cementslurry into the well so that the bottom plug may be completed at oneoperation or at one time, and the operator is assured of a. completeshut off without the necessity of making further tests.

in reading of the Y rift',

Having described only typical preferred vmanners of carrying out themethod of our invention, we do not wish to be limited or restricted tothe specific details herein set forth, but wish to reserve to ourselvesany variations or modications that may appear to those skilled in theart or -fall within the scope of the following claims.

Having described our invention, we claim:

1. The method of handling lcement slurry in a Well containing a iluidhaving an electrical conductivity diierent from prising arranging adevice in the well operable to pass an electrical current through theadjacent fluid, passing a body of the cement slurry down the well untilit passes the said device to produce alternate submersion of thedevicein the slurry and the iluid in the well, indicating the resistanceat said device, and measuring the depth of said device at the times ofthe change in resistance when said alternate submersion occurs.

2. 'I'he method of cementing a well containing a fluid having anelectrical conductivity dinerent from that of cement slurry comprisingproviding a device in the well operable to create an electrical field inthe uid in which it is submerged, indicating the inductance in saiddevice, passing a body of cement slurry down the well to the device sothat the slurry replaces the iluid in the field of the instrument,indicating the position of the device in the well, and noting theposition of the device Aat the time of change in the inductance in saiddevice when said replacement in the iield occurs.

3. 'I'he method of cementing a well containing a fluid having anelectrical conductivity diiierent from that of cement slurry comprisingarranging a device in the well operable to create an electrical capacityin a portion oi the'fiuid in which it is submerged, indicating saidcapacity, indicating the position of the said device, passing a body ofcement slurry down the well to the said device thereby producing asubstantial change in the said capacity, and observing the position ofsaid device at the time of substantial change in said capacity.

4. In a method of cementing a well containing a fluid having anelectrical conductivity different from that of cement slurry, thearrangeemnt of a device in the well operable to create an electricalresistance gap in the iluid in which it is submerged, indicating at thesurface of vthe ground resistance changesv in said gap, indicating theposition of said device by suspending the same on a metered line,passing a body of cement slurry down the well to the device to replacethe said fluid in the gap of the device with cement slurry and thuschange the resistance of the gap, and observing the position of saiddevice at the time of the said change in the resistance of said gap.

5. 'I'he method of cementing a well containing a iluid having anelectrical conductivity different from that of cement slurry comprisingproviding a device in the well operable to create an electromagneticiield .in the uid in which itis submerged, indicating changes in theinductance in said field, indicating the position of said device byhandling the same on a metered line, passing a body of cement slurrydown the well to the device to replace the fluid in the ileld of thedevice with cement slurry and thus change the inductthat of the slurrycomance in the field, and observing the position' of said device at thetime of said change in the inductance in said eld.

6. The method of cementing a well containing a iluid having anelectrical conductivity different from that of cement slurry comprisingproviding '7. 'Ihe method of cementing a Well containing4 a iluid havingan electrical conductivity dilerent from that of cement slurrycomprising providing a device in the Well responsive t`o changes in theelectrical conductivity of the iluid in which it is submerged,indicating the responsiveness oi the device, indicating the position ofthe device, passing a body of cement slurry down the well to move pastthe device so that said device is alternately submerged in the iluid inthe well and the cement slurry, and observing the position of the deviceat the time of marked change in the responsiveness of the device 8. Themethod of cementing a well containing iluid having an electricalconductivity different from that of cement slurry comprising providing aunit in the well having an electrical conductor exposed to the fluidstherein, passing an electrical current through said conductor to flowthrough the surrounding iluid, passing a body oi cement slurry downth`e--well to surround and then pass the unit, metering the-current toobserve the change therein at the time of movement of the slurry pastthe conductor, and indicating the position oi the unit at the time ofsaid change.

9. 'I'he method of cementing a well containing iluid comprising,providing a .device in the well which device is sensitive to theconductivity of the surrounding fluid to iniluence an electricalcircuit, passing a body of cement slurry into the well to surround andpass the device and thereby iniluence the circuit, and indicating theposition of the device when said circuit is influenced.

10. The method of cementing a well containing rotary mud comprisingarranging a device in the well responsive to the electrical resistivityof the fluid in which it is submerged, providing an electrical circuitcontaining said device and an electrical meter, indicating the positionof said device, introducing a body of cement slurry into the well tomove down past said device, and observing the position of said devicewhen said meter is aiected by movement` of the slurry past said device.f

ll. 'I'he method of cementing a well containing rotary mud comprisingproviding in the well a device responsive to the electrical resistivityo! the fluid in which it is submerged, providing an electrical circuit.containing said device and an

